Fluoridation of drinking water



Feb. 19, 1963 F. J. MAIER ETAL FLUORIDATION OF DRINKING WATER OriginalFiled Oct. 29, 1957 INVENTORS J Mar/1&1

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TD DRINKING FLUURSPAF? TD SIPHUN 25 EI'ULIZ Be LlH ATTORNEY UnitedStates Patent Oflfice 3,078,225 Patented Feb. 19, 1963 3,078,225FLUORIDATEQN F DRTNKING WATER Franz .l. Maier, Chevy Chase, and ErwinBeiiacir, litethesda, Md, assignors to the United States of America asrepresented by the Secretary of the Army and the Secretary of Health,Education and Welfare Continuation of application Ser. No. 693,216, 0st.29,

1957. This application Nov. 12, 1959, Ser. No. 852,581 2 Claims. (Cl.210-47) (Granted under Title 35, US. Code (1952), see. 266) Theinvention described herein, if patented, may be manufactured and used byor for the Government for governmental purposes, without the payment tous of any royalty thereon. This application is a continuation of ourcopending application Serial No. 693,216, filed October 29, 1957, nowabandoned.

This invention relates to a process and apparatus for the fluoridationof drinking water, and more particularly to a system which utilizes anaqueous solution of a reaction product of fluorspar and of awater-soluble coagulant metal salt, e.g., alum( aluminum sulfate), forthe simul taneous fluoridation of drinking water and coagulation of theimpurities contained therein.

Fluoridated drinking water is used in many communities for the purposeof supplying a sufiicient amount of fluoride ions to the diet to meetdental health standards. Water-soluble fluoride compounds are not anabundant natural product, and synthetic water-soluble fluorides are notinexpensive. Thus, sodium fluoride, which is a water-soluble fluoridenow frequently used in the fluoridation of water, costs about 14 centsper pound. Assuming. that 20 pounds of sodium fluoride are needed tofluoridate 1,000,000 gallons of drinking water, the cost of fluoridationof drinking water with sodium fluoride is $2.80 per 1,000,000 gallons.

A standard purification treatment for drinking water comprises theaddition of controlled amounts of filter alum (commercial aluminumsulfate), which effects the coagulation of impurities through the actionof the aluminum ions of the alum.

Fluorspar, which contains calcium fluoride or fluorite (CaF in naturalform, is an industrial chemical which in concentrated form containsabout 8598% calcium fluoride. It is water-insoluble and therefore cannotbe used as such for the fluoridation of drinking water. Commercialfluorspar costs about $40.00 per ton (about 2 cents per pound) and wouldrequire about 18 pounds to fluoridate a million gallons of water.

We have found that simultaneous fluoridation and coagulation of drinkingwater can be effected economically and efliciently by departing from theconventional method of supplying separate quantities of fluoride andaluminum ions to the Water. In accordance with our invention, awater-soluble reaction product is formed by contacting finely dividedfluorsp'ar with an aqueous solution of aluminum sulfate (alum). Thisreaction product is a complex which contains aluminum ions and fluorideions. This water-soluble reaction product is supplied to the drinkingWater in the form of an aqueous solution and in an amount to supplyenough fluoride ions to the drinking Water by supplementing thenatural'fluoride ion contents, if any, of the drinking water, so as toobtain the optimum fluoride ion concentration in accordance withapplicable dental health standards. The aluminum ions of theaforementioned reaction product simultaneously effect the coagulation ofimpurities contained in the drinking water supplied. While localfluoridation standards will vary, we have found that on the average, 100pounds of fluorspar is a suflicient amount of starting material toprovide a reaction product adequate for the fluoridation of 5,000,000 to6,000,000 gallons of drinking water. Thus, at an average price of $40.00per ton of fluorspar (or about $2.00 per 100 pounds), the material costfor the fluoridation of 1,000,000 gallons of drinking water isapproximately 36 cents, or about one-third of the cost of fluoridationwith sodium silicofluoride and one-eighth the cost using sodiumfluoride. The material cost of the alum needed to effect thesolubilization of the fiuorospar need not be computed separately in thisconnection because a corresponding amount of alum would have to besupplied to the drinking water in any event for coagulation ofimpurities.

As pointed out, the chemical reaction between the fluorspar and alum(aluminum sulfate) results in the formation of a water-soluble complexwhich contains aluminum ions and fluoride ions, presumably aluminumfluosulfate (AIFSOQ, accompanied by the formation of a Water-insolubleby-product, calcium sulfate (CaSO We presume that the reaction proceedsalong the general line:

A12 3 CaF C380 however, we wish it to be understood that the practice ofour invention is not predicated on the precise contents of the foregoingformula.

The reaction outlined above should not be construed to limit the use ofchemicals for dissolving fluorspar to aluminum sulfate alone. Fluorsparcan be dissolved in solutions of other chemicals used on water treatmentso that the fluoride ion is produced from fluorspar by the production ofwatersoluble fluoride complexes. For example, ferric chloride andaluminum chloride produce Insoluble complexes yielding fluoride ions insolution. addition other compounds used in water treatment such asaluminum-ammonium sulfate, aluminum potassium sulfate, chlorinatedcopper as (chlorinated ferrous sulfate), ferric sulfate, ferroussulfate, and sodium aluminate may readily beused for this purpose.Mixtures of the foregoing metalsalts with alum and/ or With each otherare of course also suitable for carrying out the purposes of ourinvention.

In order to increase the efliciency of the reaction, we have devised adissolving tank, illustrated in the accompanying drawings. We wish it tobe understood, however, that the practice of our inventionis notnecessarily limited to the use of this particularv tank, even' though weare presently not aware of any other commercially available apparatuswhich will work with equal or greater efliciency in our system. Wefurther wish it tobe understood that the dimensions of the tank may bereadily altered (increased or decreased) to accommodate larger orsmaller drinking water systems, if desired.

In the accompanying drawings,

FIGURE 1 shows, in front elevation, and

FIGURE 2 shows, partly in section and partly in side elevation, atdissolving tank in accordance with our invention.

In the drawing, 10 denotes a ground-supported stainless steel tankhaving a liquid capacity of about gallons. The bottom portion 11 of thetank is preferably generally cone-shaped and is provided with aT-fitting, 12, which is used as a clean-out. This bottom portion 11constitutes a well for the reception of comminuted,

fluorspar. A vertical open pipe 13 extends into the well portion of tank10' for the admission of an aqueous solution of alum at a controlledrate. Optionally, this pipe may be provided with height adjustmentmeans, such as a collar 14, which in turn is bolted or otherwiseconnected to a pair of channel irons 15, 15. Collar 14 may be providedwith set-screws or other adjustable means, so that the height of pipe 13may be adjusted to the de sired distance from the bottom of the cone oftank. 10.

Channel irons 15, 15 are shown in the drawing'aflixed' to the upper edge16 of cylindrical tank 10; however,

amazes 3 they may instead be provided with independent groundsupportingmeans, or fastened to a wall or ceiling.

Associated with pipe 13 is an agitator or stirrer 17 of commercialdesign, e.g., a 6 inch diameter stainless steel fiat-bladed rotaryturbine impeller. In the model shown, the agitator is rotated by meansof vertical shaft 18 which extends through pipe 13; a motor 19, e.g., alit-hp. electric motor rotates shaft 18 and may be mounted on pipe 13 bymeans of a bracket 20 (or may be independently supported). Processefiiciency demands that stirrer 17 be located proximate to the lowermouth of pipe 13. For maximum elficiency we have provided the lowermouth of pipe 13 with a flared section 21, preferably of conical shaperesembling a funnel (a bell shape or other configuration encompassingthe impeller, also being acceptable), and have located the stirrer 17within or slightly below the widened portion of flare 21. Stirrer 17 islocated at that position to promote the maximum reaction between thefluorspar and alum solution at their point of initial contact.

At the upper portion of tank 10, vertically spaced from its upper end,there is provided an annular weir 22, e.g., in the shape of a truncatedcone segment or frustocone whose base has a diameter corresponding tothe inner diameter of tank lit. Their weir 22 collects the aqueoussolution of the water-soluble reaction product produced at the site ofagitator 17 and in the fluorspar bed. This aqueous solution of thereaction product is supplied in controlled amounts to the drinking waterreservoir through outlet 23. As shown diagrammatically in FIGURE 2 ofthe drawings, the space between the surface of the linerspar bed and therim of weir 22 is filled during the process of the reaction with anupwardly moving liquid. This aqueous solution contains aluminum ions andfluoride ions together with the complex formed as a result of thereaction. In addition there are suspended in the liquid the unreactedsolid particles of fluorspar and a by-product sludge containing calciumsulfate. However, these particles do not rise to the level of the upperrim of weir 22; only the clear liquid flows into the weir.

The calcium sulfate sludge which builds up above the surface of thefluorspar bed is removed continuously or intermittently by appropriatemeans, e.g., by siphoning through a suction line 24 extending into thetank through its open upper end.

In accordance with a modification of our invention as above-described,the siphon and suction line 24 are omitted, and the constantly formingcalcium sulfate sludge is removed from the tank by increasing thewithdrawal rate of the solution of the reaction product so that theupward velocity of liquid within the tank is sufiicient to produce ahydraulic separation of the sludge from the unreacted fluorspar. By thismeans, the sludge is carried into the flocculation basin of the watertreatment plant along with the solution of the reaction products. Thissludge then adds to the turbidity of the untreated, incoming water andthereby assists in its coagulation by providing additional nuclei forthe formation of due. The particles of sludge introduced in this mannerare subsequently removed from the water by settling or filtration.Certain obvious design changes in the construction of dissolving tank10, to accomplish the simultaneous withdrawal of the calcium sulfatesludge together with the aqueous solution of the fluoride-ion containingreaction product, may be made, but are not illustrated.

The fluorspar consumed in the reaction is replenished through the top oftank 10. The rate of the reaction is adjusted by feeding appropriateamounts and concentrations of aluminum sulfate (alum) in the form of anaqueous solution through pipe 13. This rate of supply and concentrationcan be determined through relatively simple stoichiometricalcomputations. Fitting 12 is used for periodic removal of solidimpurities which drop through the fluorspar bed into well 11; theseimpurities are principally those contained in the fluorspar supplyitself, which impurities settle out as the reaction proceeds.

in an actual run in the apparatus described, 300 pounds of commiuutedfluorspar of about 97% purity, were placed into the dissolving tankcontaining the alum solu tion approximately to the upper level ofconical well section 11, and a solution of 50 pounds of alum (aluminumsulfate) in 50 gallons of water corresponding to a solution strength ofabout 11% was gradually and accurately proportioned into the tankthrough the vertical pipe 13. With the agitator running, the alumsolution reacted with the fluorspar and a water solution of thealuminum-ionand fluoride-ion-containing water-soluble reaction product(presumably aluminum fluosulfate or analogous fluoridealuminum complex)ascended above the fluorspar bed. The liquid collected in the weir abovethe fluorspar bed was fed into the water reservoir at a maximumdesirable rate of 555 ml. per minute for the tank shown. The feed ofalum into the dissolving tank described was adjusted to maintain asubstantially constant level of reactionproduct solution in the weir. Amoist sludge of Waterinsoluble by-product (presumably calcium sulfate)accumulated below the weir, and was withdrawn at the time of fluorsparreplenishment through the top of the tank by means of suction line 24and siphon. The fluorspar used up in the reaction was replenished fromtime to time in 100 lb. increments through the top of the tanksubstantially immediately after the withdrawal of sludge. It was foundthat at the foregoing rates of supply, 100 lbs. of fluorspar wassufficient to furnish a reaction product for the fluoridation of6,000,000 gallons of water at a concentration of 1.0 mg./ l. offluoride.

It is obvious that adjustments can be made in the foregoing procedure,e.g., by increasing the rate of Withdrawal of the liquid solution of thewater-soluble reaction product and simultaneous increase in the rate ofsupply of alum solution and for adjustment of the range of concentrationof the alum solution, e.g., within the solubility range of aluminumsulfate. However, a stoichiometric excess of fluorspar should generallybe maintained for process efliciency.

Likewise, if it should be found in actual operation that the amount ofaluminum ions supplied to the drinking water by means of the feed of thewater solution of the reaction product is insufficient to coagulate theimpurities contained in the drinking water, additional alum can be fedinto the water supply, e.g., by a separate feeder.

As pointed out, the size and relative proportions of the tank are forpurposes of illustration only and may be varied within broad limitswithout departing from our invention.

It will thus be seen that we have provided a system of simultaneousfluoridation and coagulation of impurities in drinking water by the useof inexpensive inorganic chemicals, and without the need for complicatedmachinery.

Although we have illustrated and described the preferred form of ourinvention, it will be obvious that various changes may be made thereinwithout departing from the spirit of the invention or from the scope ofthe subjoined claims.

We claim: 1. A continuous process for fluoridation of drinking water,comprising:

continuously feeding and stirring an aqueous solution of an aluminumsulfate salt into a bed of comminuted fluorspar, whereby a water-solublereaction product containing aluminum ions and fluoride ions is formed asan aqueous solution, and a substantially water-insoluble calcium sulfatesludge is formed as a by-product;

continuously supplying measured amounts of said aluminum ions andfluoride ions containing aqueous solution of said reaction producttogether with cal- 5 6 oiurn sulfate sludge by-product to a supply ofdrink- References Cited in the file of this patent ing Water, saidamounts being adequate to provide UNITED STATES PATENTS sufiicientfluoride ions to meet dental health standards and to provide at leastpart of the aluminum ions iggg Travers i Z necessary to effectcoagulation of the impurities in 5 77 Travars 1944 said drinking water,and said calcium sulfate sludge i g et a g 1944 assisting in thepurification of said drinking water by 2495371 ZZ: 3; 1947 ,d, r

21 52333? of the mpunues comm m dmnk 2,429,315 Green Oct. 21, 19472,643,226 White June 23, 1953 and replenishing said fluorspar bed with afresh supply 10 2,678,915 Kaunske m May 18 1954 of comminuted fiuorsparto replace reacted fluorspar. 2. Process according to claim 1, whereinsaid calcium OTHER REFERENCES sulfate sludge is hydraulically separatedfrom said fluor- Water and Sewage Works (periodical), vol. 97, No. 4,spar by the flow of said last-named aqueous solution. April 1950, pages143-445.

1. A CONTINUOUS PROCESS FOR FLUORIDATION OF DRINKING WATER, COMPRISING:CONTINUOUSLY FEEDING AND STIRRING AN AQUEOUS SOLUTION OF AN ALUMINUMSULFATE SALT INTO A BED OF COMMINUTED FLUORSPAR, WHEREBY A WATER-SOLUBLEREACTION PRODUCT CONTAINING ALUMINUM IONS AND FLUORIDE IONS IS FORMED ASAN AQUEOUS SOLUTION, AND A SUBSTANTIALLY WATER-INSOLUBLE CALCIUM SULFATESLUDGE IS FORMED AS A BY-PRODUCT; CONTINUOUSLY SUPPLYING MEASUREDAMOUNTS OF SAID ALUMINUM IONS AND FLUORIDE IONS CONTAINING AQUEOUSSOLUTION OF SAID REACTION PRODUCT TOGETHER WITH CALCIUM SULFATE SLUDGEBY-PRODUCT TO A SUPPLY OF DRINKING WATER, SAID AMOUNTS BEING ADEQUATE TOPROVIDE SUFFICIENT FLOURIDE IONS TO MEET DENTAL HEALTH STANDARDS AND TOPROVIDE AT LEAST PART OF THE ALUMINUM IONS NECESSARY TO EFFECTCOAGULATION OF THE IMPURITIES IN SAID DRINKING WATER, AND SAID CALCIUMSULFATE SLUDGE ASSISTING IN THE PURIFICATION OF SAID DRINKING WATER BYFLOCCULATION OF THE IMPURITIES CONTAINED IN SAID DRINKING WATER. ANDREPLENISHING SAID FLUORSPAR BED WITH A FRESH SUPPLY OF COMMINUTEDFLUORSPAR TO REPLACE REACTED FLUORSPAR.